Process for liquefying natural gas containing nitrogen

ABSTRACT

Natural gas containing nitrogen is cooled and liquefied by a cascade system having two closed refrigeration circuits operating on multicomponent refrigerant mixtures. The liquefied gas is distilled into three fractions: nitrogen, a mixture of nitrogen and methane, and liquid natural gas substantially free of nitrogen. The mixture of nitrogen and methane provides the refrigerant in a third and coldest open circuit of the cascade system to subcool the liquid natural gas to storage temperature.

United States Patent Martin Stretch Nieder-Eschhach, Germany 784,124

Dec. 16, 1968 Aug. 3, 1971 Maser Grlesbeim GmbH Frankfurt, Germany Dec.20, 1967 Germany inventor Appl. No Filed Patented Assignec PriorityPROCESS FOR LIQUEFYING NATURAL GAS CONTAINING NITROGEN 6 Claims, 1Drawing Fig.

user. sz/za, 62/40 F25jl/02, F25j3/02 FieldofSearch sznnza, 24,27. 28,40

Int. Cl.

References Clted UNITED STATES PATENTS 3/1950 Cooper 62/60 m'rumu.connmme N;

2,960,837 2/1960 Swenson 62/40 3,020,723 2/1962 De Lury 62/40 3,218,81611/1965 Grenier... 62/28 3,254,496 6/1966 Roche..... 62/40 3,254,4956/1966 Jackson 62/40 OTHER REFERENCES Kleemenko A. P., One Flow CascadeCycle in Progress in Refrigeration Science and.Technology Pergamon Press1960 p 34-39 Primary Examiner- Norman Yudltoff Assistant ExaminerArthurF. Purcell Attorney- Paul W. Garbo ABSTRACT: Natural .gas containing;nitrogen is cooled and liquefied by a cascade system having two closedrefrigeration circuits operating on multicomponeznt refrigerantmixtures. The liquefied gas is distilled into three fractions: nitrogen,a mixture of nitrogen and methane, and liquid natural gas subatantiallyfree of nitrogen. The mllxture of nitrogen and methane provides therefrigerant in a third and coldest open circuit of the cascade system tosubceol the liquid natural gas to storage temperature.

PATENTEU AUG 3mm INVENTOR.

MARTIN STREICH AGENT I wzizoo m EmPEz PROCESS FOR LIQUEFYING NATURAL GASCONTAINING NITROGEN BACKGROUND OF THE INVENTION This.invention relatesto a process for the liquefaction of natural gas containing nitrogen. incountercurrent heat exchangers by means of closed circuits ofmulticomponent refrigerant mixtures, arranged in cascade manner. Dutchpatent 108,678 discloses that natural gas can be cooled this way down toits storage temperature, approximately -l60 C. By using multicomponentrefrigerant mixtures which, preferably. are made up ofcomponents of thenatural gas, it is possible to operate with a very small number ofcascade circuits. It is possible to accomplishthe total liquefaction andsubcooling of the natural gas by means of only two circuits havingmulticomponent refrigerant mixtures, each of which mixtures goes throughonly one pressure stage. However, thermodynamically, it is not favorableto extend the temperature ranges of theindividualrefrigeration circuitstoo much. It is therefore advisable to use a third circuit. However, anadditional circuit ordinarily requires its own compressor as well as thenecessary control and regulating devices.

It is a principal object of this invention to simplify the thirdrefrigeration circuit and thereby lower investment and operating costsfor the liquefaction and subcooling of nitrogen-containing natural gas.

SUMMARY OF THE INVENTION In accordance with this invention, natural gashaving an appreciable content of nitrogen is cooled, liquefied andsubcooled in countercurrent heat exchangers which include closedcircuits of multicomponent refrigerant mixtures, arranged in cascademanner, each circuit going through only one pressure stage. Aftercompletion of liquefaction and prior to subcooling the liquefied naturalgas to storage temperature. the lower-boiling inert components,particularly nitrogen, are removed therefrom by distillation in acolumn. In order to sub cool the liquefied natural gas to storagetemperature, an at least partially gaseous mixture of methane andnitrogen is withdrawn from the column, liquefied by heat exchange withthe multicomponent refrigerant mixture of the colder closed circuit inthe cascadesystem, subcooled against itself in expanded condition, andfinally expanded; this expanded stream then subcools the liquefiednatural gas and is desirably also used to cool the vapor in the topportion ofthe column.

After its expansion and the transfer of its refrigeration, the gaseousmixture of the added and coldest refrigeration circuit in the cascadesystem can be used for the generation of heat and energy. Inasmuch asthis added circuit is an open circuit, no compressor is needed andthereby-investment and operating costs are reduced.

In a preferred embodiment of this invention, the multicomponentrefrigerant mixture of the colder closed circuit is first cooled in thesump of the nitrogen distillation column. Then, it is expanded and usedto subcool the liquefied natural gas to a temperature of about l40 C. Ifdesired, before expanding the refrigerant mixture of the colder closedcircuit, it may be further cooled by passage through a coil in themiddle portion of the distillation column. l-Urthermore, the nitrogenwithdrawn from the top of the column and the vent gas from the liquidnatural gas storage tank can be utilized as sources of refrigeration inthe heat exchangers used to liquefy and sub cool the liquefied naturalgas. It is advantageous to expand each of the multicomponent refrigerantmixtures in the closed circuits to a pressure between about 3 and 5atmospheres ab solute instead of atmospheric pressure since, in thisway, the pressure drop in the circuit is smaller, i.e.. energy is savedand less heat exchanger surface is needed.

DESCRIPTION OF A PREFERRED EMBODIMENT The accompanying drawing is aflowsheet illustrating a preferred embodiment of this invention.

Natural gas with an appreciable content of nitrogen is supplied throughline 1 at high pressure and approximately ambient temperature to heatexchangers 2 and 3 wherein it is cooled and liquefied. Thence, theliquefied natural gas flows through first sump coil 5 to heat the liquidin the sump of nitrogen distillation column 6, and thereby to be furthercooled. it is then expanded through expansion valve '7 and dischargedinto column 6. Column 6 operates at a pressure of 14 atmospheresabsolute. Therein, the natural gas is separated into three fractions.Substantially nitrogen-free, liquid natural gas is withdrawn from thesump of column 6. Substantially pure nitrogen is withdrawn from the topof column ti. From the middle portion of column ti, a mixed methane andnitrogen fraction is withdrawn, its volume and composition beingcontrolled so as to cool the liquid methane to a storage temperature of--l60 C. The nitrogen-free liquid natural gas is withdrawn from the sumpof column 6 through line 8 and fiows through the two heat exchangers tand 9 where it is subcooled to 160 C. Thereafter, the liquid natural gasis expanded and discharged into a storage tank.

Nitrogen withdrawn from column 6 through line 10 fiows successivelythrough heat exchangers 9, 4, 3 and 2, and thus is heated to ambienttemperature.

The first and warmer closed circuit supplies practically all therefrigeration needed between ambient temperature and approximately 75 C.in this circuit, the refrigerant mixture of methane, ethane, propane andbutane is completely liquefied at ambient temperature when compressed to42 atmospheres absolute. This mixture is compressed to 42 atmospheresabsolute by compressor 10, is completely liquefied in cooler 11 by meansof cooling water, and flows through line 12 into heat exchanger 2wherein it is subcooled against itself after having been expanded. Afterexpansion to 3 atmospheres absolute in expansion valve 13, thisrefrigerant mixture trans fers its refrigeration to the natural gas andthe refrigerant mixture of the second and colder closed circuit passingthrough heat exchanger 2. Line 14 returns the refrigerant of the firstcircuit from heat exchanger 2 to compressor 10 and thus closes thecircuit.

The multicomponent refrigerant mixture of the second and colder closedcircuit flows through line 15 and heat exchangers 2 and 3 whereby it iscooled and liquefied. This liquid then flows through second sump coil 16in column s and through heat exchanger 4. The thus subcooled liquid isexpanded in expansion valve 17 and passed through line to which extendssuccessively through heat exchangers t, 3 and 2, whereby the mixture iswarmed to ambient temperature. Line 118 returns the warmed mixture tocompressor 119 where it is again compressed and, after flowing throughwater cooler 20, passes through line 15.

At selected operating conditions of composition and pressure afterexpansion, the refrigerant mixture of the second circuit can transferpractically no refrigeration below C. to the natural gas. it ispossible, but thermodynamically not favorable, to extend the operationof this second circuit over an even greater temperature range. Rather,in accordance with this invention, the second circuit is followed by athird circuit in the cascade arrangement. This third circuit which isopen operates with a mixture of nitrogen and methane to supply thenatural gas with refrigeration between 145 C. and C., needed forsubcooling. As already mentioned, the methane and nitrogen mixture forthis added circuit is withdrawn from the middle portion of column ti,liquefied against the expanded, evaporating refrigerant mixture of thesecond circuit and, after expansion, vaporized by heat exchange with thenatural gas. Thus, refrigeration is essentially supplied by athree-circuit cascade system with the following characteristics:

Transfer ofrefrlger- Circuit Rwillyvi'rlllt mixtnn: ation at typ l ih anll-;, CH1 +30 i -T5 C. ClOStd. b t. h- N; -75 i0 '-145 C. D0. tllt. l45to 160 C Opt-n.

For the third and open circuit, a gaseous mixture of methane andnitrogen is withdrawn from column 6 through line 21 and liquefied inheat exchanger 4 with refrigeration from the second circuit. The liquidthen flows through heat exchanger 9 where it is subcooled, and throughexpansion valve 22 where it is expanded to 3.8 atmospheres absolute.This expanded stream gives up a part of its refrigeration in coolingcoil 23 of column 6 in order to produce reflux. This stream passesthrough line 24 back to heat exchanger 9 where it gives up the greaterpart of its refrigeration to subcool the liquid natural gas to -l60 C.it then continues to flow through heat exchangers 4, 3 and 2. whereby itis heated to ambient temperature and subsequently utilized for thegeneration of heat and energy.

The process of this invention permits an advantageous combination ofnitrogen removal from the natural gas, which is highly desirable, and athird circuit in a cascade system. It would be thennodynamicallyunfavorable to extend the temperature ranges of the first two closedcircuits too much. This is avoided by the present invention withoutnecessitating an extra compressor and its control and regulating devicesfor an additional refrigeration circuit.

What i claim is:

l. A process for liquefying natural gas containing nitrogen and suppliedat high pressure, which comprises cooling and liquefying said naturalgas by countercurrent heat exchange with two closed refrigerationcircuits in cascade arrangement,

each of said closed circuits having a multicomponent refrigerant mixturewhich undergoes a single expansion, expanding the liquefied natural gasand discharging it into a rectifying zone maintained at an intermediatepressure. withdrawing an at least partially gaseous mixture of nitrogenand methane from the middle portion of said rectifying zone to providethe refrigerant mixture for a third and coldest open circuit in saidcascade arrangement withdrawing liquefied natural gas substantially freeof nitrogen from the bottom of said rectifying zone and subcooling it tostorage temperature by countercurrent heat exchange with said opencircuit.

2. The process of claim 1 wherein the refrigerant mixture of the thirdcircuit, upon being expanded, flows through a heat exchange passage inthe top portion of the rectifying zone to condense vapor in said topportion and thus provide reflux for said rectifying zone.

3. The process of claim 1 wherein the multicomponent refrigerant mixtureof the colder of the two closed refrigeration circuits flows through aheat exchange passage in the bottom portion of the rectifying zone and,after being expanded, subcools liquefied natural gas substantially freeof nitrogen which is withdrawn from the bottom of said rectifying zone.

4. The process of claim 1 wherein substantially pure nitrogen iswithdrawn from the top of the rectifying zone and is passed incountercurrent heat exchange with the liquefied natural gassubstantially free of nitrogen withdrawn from the bottom of saidrectifying zone and with the natural gas containing nitrogen prior toits discharge into said rectifying zone.

5. The process of claim I wherein each of the two multicomponentrefrigerant mixtures is expanded to a pressure higher than 2 atmospheresatmospheres absolute.

6. The process of claim 1 wherein each of the two multicomponentrefrigerant mixtures is made up of components of the natural gascontaining nitrogen.

absolute but not exceeding about 5

2. The process of claim 1 wherein the refrigerant mixture of the thirdcircuit, upon being expanded, flows through a heat exchange passage inthe top portion of the rectifying zone to condense vapor in said topportion and thus provide reflux for said rectifying zone.
 3. The processof claim 1 wherein the multicomponent refrigerant mixture of the coldeRof the two closed refrigeration circuits flows through a heat exchangepassage in the bottom portion of the rectifying zone and, after beingexpanded, subcools liquefied natural gas substantially free of nitrogenwhich is withdrawn from the bottom of said rectifying zone.
 4. Theprocess of claim 1 wherein substantially pure nitrogen is withdrawn fromthe top of the rectifying zone and is passed in countercurrent heatexchange with the liquefied natural gas substantially free of nitrogenwithdrawn from the bottom of said rectifying zone and with the naturalgas containing nitrogen prior to its discharge into said rectifyingzone.
 5. The process of claim 1 wherein each of the two multicomponentrefrigerant mixtures is expanded to a pressure higher than 2 atmospheresabsolute but not exceeding about 5 atmospheres absolute.
 6. The processof claim 1 wherein each of the two multicomponent refrigerant mixturesis made up of components of the natural gas containing nitrogen.